It has been said that camera-equipped mobile telephones, digital cameras, digital camcorders, etc., will in the future have definitions as high as those of HDTVs, and they have been downsized for added values. However, downsizing the optical system and the image sensor will encounter the fundamental image-capturing limitations such as the sensitivity and the lens diffraction limit, and increasing the definition is expected to reach its limit in the future. Then, it will be effective to improve the image quality by adding information quantities relating to various physical characteristics used for image generation in computer graphics in addition to the insufficient image information of the subject obtained. For this, one needs to go over the limits of the conventional 2-dimensional image processing and to obtain 3-dimensional shape information of the subject and obtain physical information from the image generation process such as information of the light source illuminating the subject.
In order to input shape information, there is needed a distance-measuring system such as a range finder that emits laser light or an LED light source or a differential stereo vision system. However, such a distance-measuring system has limitations, e.g., data can be obtained for a distance of only up to several meters between the camera and the subject, and applicable subjects are limited to solid and bright diffusive objects, in addition to being a large-scale system. Then, it cannot be used for capturing images in an outdoor scene at a long distance, such as at a children's field day, or for capturing images of people, where hair and clothes are important.
Moreover, these methods obtain depth information as the 3-dimensional shape information. However, normal information, but not depth information, is very effective as physical information from the image generation process. Needless to say, it is a widely-used technique to obtain normal information from depth information obtained by these methods (e.g., Non-Patent Document 1), but it is known in the art that the precision of normal information so obtained is insufficient (e.g., Non-Patent Document 2).
That is, in order to achieve the above object with outdoor scenes or general subjects, there is needed a method for directly sensing (measuring) normal information in a completely passive manner.
Known methods for directly measuring normal information include a so-called “shape from shading” method such as a photometric stereo, and a method utilizing polarization. However, the former is an active sensing method, and cannot be called a passive sensing method, because the light source position needs to be known or the light source position needs to be varied.
As a method utilizing polarization, Patent Document 1, for example, discloses a method for generating local normal information of a subject by observing the specular reflection component while rotating a polarizing plate attached in front of the camera lens without making a special assumption on the subject illumination (random polarization: non-polarized lighting). A surface normal of a subject has two degrees of freedom, and the normal is determined by obtaining two angles: one being the incident plane that contains the incident light and the reflected light, and the other being the angle of incidence within the incident plane. Information of the incident plane of the specular reflection component is obtained from the angle of a rotated polarizing plate at which the intensity takes the minimum value.
Non-Patent Document 3 obtains, among the normal information of the subject, the angle (one degree of freedom) of the emittance plane that contains the incident light and the reflected light, by a method of observing the diffuse reflection component while rotating a polarizing plate attached in front of the camera lens without making a special assumption on the subject illumination (random polarization: non-polarized lighting). Information of the emittance plane of the diffuse reflection component is obtained from the angle of a rotated polarizing plate at which the intensity takes the maximum value.
Non-Patent Document 4 obtains the angle (one degree of freedom) by a method similar to that of Non-Patent Document 3, and further obtains the normal direction vector by directly utilizing the degree of polarization.
Moreover, Non-Patent Document 5 measures the normal information even with a texture-less subject by combining the polarization information and the shape from shading method.
Citation List
Patent Literature
[Patent Document 1] U.S. Pat. No. 5,028,138
Non-patent Literature
[Non-Patent Document 1] H. Hoppe, T. DeRose, T. Duchamp, J. McDonald, and W. Stuetzle, “Surface reconstruction from unorganized points,” Computer Graphics (SIGGRAPH '92 Proceedings), pp. 71-78, 1992.
[Non-Patent Document 2] Tomoaki Higo, Daisuke Miyazaki, Katsushi Ikeuchi, “Simultaneous Determination of Shape and Reflection Parameters from Shading”, Meeting on Image Recognition and Understanding (MIRU2007), pp. 1093-1098, 2007
[Non-Patent Document 3] Ondfej Drbohlav and Sara Radim, “Using polarization to determine intrinsic surface properties”, Proc. SPIE Vol. 3826, pp. 253-263, 1999
[Non-Patent Document 4] G. A. Atkinson and E. R. Hancock, “Recovery of surface orientation from diffuse polarization,” IEEE Transactions of Image Processing, Vol. 15, No. 6, pp. 1653-1664, 2006
[Non-Patent Document 5] Gary A. Atkinson and Edwin R. Hancock, “Shape Estimation Using Polarization and Shading from Two Views”, IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 29, Iss. 11, pp. 2001-2017, 2007